Entecavir, [1-S-(1α,3α,4β)]-2-amino-1,9-dihydro-9-[4-hydroxy-3-(hydroxymethyl)-2-methylenecyclopentyl]-6H-purin-6-one, is currently used for treating hepatitis B virus infection, whose structure is composed of a cyclopentane ring having purine, exomethylene, hydroxymethyl, and hydroxy substituents at the 1S-, 2-, 3R-, and 4S-positions, respectively. There have been conducted a number of studies to develop methods for preparing entecavir.
For example, U.S. Pat. No. 5,206,244 and WO 98/09964 disclose a method for preparing entecavir shown in Reaction Scheme 1:

The above method, however, has difficulties in that: i) the cyclopentadiene monomer must be maintained at a temperature lower than −30° C. in order to prevent its conversion to dicyclopentadiene; ii) residual sodium after the reaction as well as the sensitivity of the reaction toward moisture cause problems; iii) the process to obtain the intermediate of formula a) must be carried out at an extremely low temperature of below −70° C. in order to prevent the generation of isomers; iv) a decantation method is required when (−)-Ipc2BH (diisopinocampheylborane) is used for hydroboration; v) the process of the intermediate of formula a) does not proceed smoothly; and, vi) separation by column chromatography using CHP-20P resin is required to purify entecavir.
WO 2004/52310 and U.S. Pat. Publication No. 2005/0272932 disclose a method for preparing entecavir using the intermediate of formula (66), which is prepared as shown in Reaction Scheme 2:

The above preparation method of the intermediate of formula (66) must be carried out at an extremely low temperature of −70° C. or less, and the yield of the desired product in the optical resolution step is less than 50%.